In many lighting systems it is desirable to have the ability to change the light intensity level (i.e., dim the light) over a wide range of intensities. A number of approaches to dimming have been utilized conventionally. One approach for AC-powered systems is the use of a phase-cut technique, in which portions of the AC signal driving the light emitter (for example, an incandescent lamp) are progressively zeroed, resulting in progressively less power being applied to the light emitter and thus a reduction in the light intensity. Phase-cut systems are typically not directly applicable to DC-driven light emitters such as light-emitting diodes (LEDs). In particular, LED-based lighting systems often have drivers that require a minimum level of power, and if the power is reduced through phase-cut dimming, the driver is starved for power at low dimming (light) levels, resulting in inefficiency, inability to operate, and/or flickering of the light.
Furthermore, for larger lighting systems, it is desirable to provide an interface to a control system that enables the use of one control protocol to address multiple luminaires (or other illumination systems). Such systems (for example, Dali or DMX) typically use a control signal separate from the power supply, for example a 0-10V signal where the value of the voltage signifies the desired light intensity level or a digital dimming signal. In such a system, the light intensity level may be controlled by varying the power to the light-emitting element in a number of ways. In one approach, the current or voltage level supplied to the lighting elements is varied in response to the control signal. In another approach, the power to the lighting elements is modulated, that is, the duty cycle—i.e., the fraction of a signal period during which the signal is non-zero—is varied in response to the control signal. This is similar to phase cut dimming, but in this case full power is applied to the lighting power supply and its output is modulated in response to the control signal. In some approaches the analog control signal is converted to a digital signal, which is then converted to a light intensity level. The human eye is relatively sensitive to changes in light intensity, and also is not linearly responsive to light intensity levels. Thus, basic control systems may achieve the ability to change the light intensity level, but produce undesired visual effects such as flicker, obvious “steps” in the intensity level, and/or response times that are too slow or too fast. Basic control systems also may not provide the desired controllability at low light intensity levels. This is particularly true for systems that require fine control of the light level, for example the ability to dim the power to the lighting elements or the light intensity to 5% or 1% of the full-scale (i.e., maximum) value.
For example, consider the case of a 0-10 V analog control input being converted to a digital signal by an analog-to-digital converter (ADC) and then applied to the output by means of an n-bit pulse-width-modulation (PWM) generator. The minimum step level (i.e., the ADC resolution) is generally defined as 10V/(2n−1). Many common systems, for example a microcontroller used in the dimming system, utilize a 10-bit PWM generator. This results in 1024 steps, which means each step is about 100%/1024, or about 0.1%, of full scale.
If the output of the ADC is directly applied, a full-scale change in input signal, for example from off to full on (0V to 10V), will immediately send a full power signal to the lighting system. This may present several problems. First, it creates an immediate change in light intensity that may be faster than desired. From a visual perspective, it is preferable to have a smooth transition from one light intensity level to another, rather than a discontinuous step transition. Additionally, a full power signal imposes a very large step load on the power supply of the system, which may potentially damage the power supply or reduce its lifetime, or it may cause a temporary change in output voltage before it recovers, and this output voltage change may result in an unintended intensity change of the load.
One way to eliminate instantaneous changes is to sample the control signal and provide a rolling average of the sampled values to the system. If the sample period is τs with ns samples in the rolling average, then it will require a time of τs×ns for a change to fully propagate through the ADC. For example, if the sample period is 25 milliseconds (ms) and the number of samples in the rolling average is 32 it will take 25×32=800 ms for the light to complete its change in intensity.
However, in this case the minimum step level change in the light output is now determined by the change in input to the ADC divided by the number of samples rather than by the ADC resolution. The situation where the step size is largest and most visible is if the system is changed from off to full power or vice versa. In this case, the input is changed by 100%, and the step size presented to the PWM generator is 100%/32=3.125%. This is a relatively large change in intensity, which may be visible as unacceptable discrete steps (or “steppiness”) in the light intensity. Herein, steppiness of emitted light is defined as the presence of discontinuous jumps (up or down) in intensity during changes in intensity (e.g., dimming) that are visibly apparent to the human eye. Conversely, light intensity changes are “smooth” or “substantially smooth” if lacking such visibly apparent discontinuous changes in intensity.
The steppiness may be reduced by increasing the number of samples ns. However, this has the disadvantage of increasing the response time. If ns is increased by a factor of 6 to reduce the step size from 3.125% to about 0.5%, then the response time will increase by a factor of 6 to about 4.8 seconds, which may be undesirably long. Decreasing the sample period and increasing the number of samples may also reduce steppiness; however, this approach generates a large number of samples that need to be stored in the controller memory. Many low-cost controllers lack sufficient memory to accommodate this approach.
In view of the foregoing, a need exists for systems and techniques enabling the smooth, visually pleasing, flicker-free, and accurate change in light intensity in lighting systems with appropriate response times.